Electric alarm system, preferably for fire alarms



Dec. 27, 1966 s. s. MEYER 3,295,121

ELECTRIC ALARM SYSTEM, PREFERABLY FOR FIRE ALARMS Filed Dec. 27, 1962 ALARM DfV/CE United States Patent Ofiice Patented Dec. 27, 1966 3,295,121 ELECTRIC ALARM SYSTEM, PREFERABLY FOR FIRE ALARMS Svend Scheel Meyer, Copenhagen Herlev, Denmark, assignor to Danske Securitas A/S, Copenhagen V., Denmark, a Danish corporation Filed Dec. 27, 1962, Ser. No. 247,694

Claims priority, application Denmark, Dec. 30, 1961,

5,263/61 3 Ciaims. (U. 340-237) The invention relates to an electric alarm system, preferably for fire alarms, and of the kind consisting of two metallic ionizing chambers connected in series and each provided with a radioactive radiating member, one of the said chambers being closed to the ambient atmosphere whereas the other chamber is open and communicates with both the first chamber and the room to be watched, the walls of the two chambers being each connected to its separate pole of a source of voltage, and a voltage sensitive indicating member being connected to electrodes in the two chambers.

Electric alarm systems of the aforesaid kind are known and they are based on the phenomenon that the electric current which from the voltage source may be forced through the ionizing chambers depends on whether the ions transmitting current are formed in pure gases or in gases that are mixed with smoke or liquid particles. In the last mentioned case a part of the ions produced are deposited on the particles of the smoke or liquid, the said particles being of greater mass and moving at a substantially lower velocity than the gas ions. An amount of gas to which are added particles of smoke or liquid will thus convey a smaller amount of electric current than would the corresponding amount of pure gases. If smoke or liquid particles enter into the open ionizing chamber of such a system, the electric resistance in the said chamber will be increased whereby the potential relationship between the two series-connected ionizing chambers is altered which will cause the voltage sensitive indicating member to react.

Practice has disclosed that it is very difi'icult to ensure such a balance of the current conditions in the two seriesconnected ionizing chambers that the installation becomes sufficiently reliable in operation, in particular if a radioactive radiating member is used in each of the two ionizing chambers. It is furthermore diflicult to obtain so good and substantially symmetrical insulating conditions in the two chambers that the conditions of ionization, even without particles of smoke or liquid, are sufficiently uniform.

These drawbacks are eliminated in a system according to the invention thereby that the ionizing chambers are connected by a joint diaphragm of electrically insulating material in which a slender radiating member is so disposed as to project into both ionizing chambers, the electrodes consisting of a metal grid surrounding the radiating member along at least a part of its length.

As a result, a variation in the strength of the active radiation from the radiating member will act upon both chambers in the same proportion, whereby the system will be reliable in operation.

In one embodiment of the system according to the invention the radiating member consists of a glass tube filled with a radioactive gas such as krypton, which will ensure a uniform distribution of the active radiation throughout the length of the radiating member.

In another embodiment of the system according to the invention the radiating member is disposed in such manner that it is displaceable in its longitudinal direction, as a result of which corrections to compensate for a geometrical or material difierence between the two ionizing chambers may be made by displacement of the radiating member.

In a third embodiment of the system according to the invention the metal grid is connected to a tap of a capacitive voltage-dependent voltage divider the extreme ends of which are connected each to their separarte ionizing chamber and which constitute one branch of a bridge arrangement that is fed with current, partly from the direct current source, partly from an alternating current source which constitutes the other branch of the bridge arrangement, the indicating member being inserted between the said tap and a point'of the alternating current voltage source, which point is balanced in relation to the said extreme ends.

As a result, the internal resistances of the ionizing chambers will normally determine the direct voltage across each of the two voltage-dependent capacitors of the voltage divider, whereas a variation in the internal resistance of the open ionizing chamber will produce a variation in the direct voltage of the capacitor connected in parallel with this chamber, which will involve a substantial non-balance in the normally balanced bridge arrangement, whereby the system becomes very sensitive to particles of smoke or liquid entering the open ionizing chamber.

In a preferred embodiment of the system the indicating member may consist of an amplifier in the output circuit of which there is inserted a signal relay or a similar alarm device.

The invention will now be further described with reference to the drawing, in which FIGURE 1 shows an embodiment of an electric alarm system according to the invention, and

FIGURE 2 a modification of same.

In the drawing, 1 and 2 are ionizing chambers having a joint diaphragm 3 of electrically insulating material in which there is provided a slender radiating member 4 in such manner that it projects into both of the chambers 1 and 2. The one ionizing chamber 1 is closed to the ambient atmosphere, whereas the other chamber 2 is open both to the first chamber and the atmosphere, the wall of the chamber being, for example, provided with perforations 5 through which the chamber communicates with rooms to be watched for any occurring particles of smoke or liquid. The walls of the two chambers are at points 6 and 7 connected by terminals 8 and 9 each to their separate pole 10 and 11 of a voltage source 12, for example a storage battery, which is constantly impressing the ionizing chamber with a positive voltage of about 200 volts, whereas the other pole 11 of the battery and and consequently the terminal 9 may be earthed. The radiating member may according to the invention be a glass tube filled with a radioactive gas as krypton. The radiating member 4 is surrounded at least along a part of its length by a metal grid 13 which is connected to a voltage sensitive indicating member which in the em bodiment according to the invention that is shown in FIGURE 1 consists of a cold-cathode tube 14, the cathode 15 of which is connected to the negative pole 11 of the battery 12, its anode 16 being connected to the positive pole 10 of the battery, whereas the control grid 17 of the tube is connected to the metal grid 13 across a terminal 18. During normal operation of the system the gas in the two chambers will be ionized by the radiating member 4- and the ions produced will charge the metal grid 13 and thereby control the grid 17 to such potential that the cold-cathode tube 14 is non-conductive. A signal relay 19 inserted in series with the anode 16 of the tube receives no current in this condition and is therefore not in its attracted position. The radiating member 4 is according to the invention disposed in such manner that it is displaceable in its longitudinal direction and it is adjusted in such manner that a slight decrease in the conditions of ionization in the open chamber 2 will cause such an increase in the grid voltage of the tube that the tube becomes conductive, whereby the signal relay 19 receives current and alarms. If particles of smoke occur in the room to be watched, the particles will enter the open ionizing chamber 2, and these particles will absorb a number of the ions in the said chamber, and since the smoke particles are of relatively great mass, the charged particles of smoke will move at lower velocity than the free ions so that the ionizing current in the open chamber is reduced, whereby the voltage of the control grid 17 is increased and the tube 14 bursts into glow, after which the signal relay 19 attracts and alarms. For example, a visible signal such as a lamp mounted near the ionizing chambers may be connected to the contacts of the relay 19 so as to indicate which of a plurality of systems has reacted to any particles of smoke or liquid or egg. has reacted owing to a fault.

It will be appreciated by those skilled in the art that both the cold-cathode tube 14 and the signal relay 19 'may be substituted by other devices responsive to voltage and current, such as transistors, without falling outside the scope of the invention.

In the embodiment of the invention disclosed in FIG- URE 2, the metal grid 13 is connected across the terminal 18 to a tap 20 of a voltage divider which consists of two voltage dependent capacitors 21 and 22 the outer terminals 23 and 24 of which are connected to the closed ionizing chamber 1 across the terminal 8 and to the open ionizing chamber 2, respectively, across the terminal 9. The ionizing currents in the two chambers will in that case act as leak resistances between the terminals 8, 18 and 9, 18, which is indicated in FIGURE 2 by dotted resistances 25 and 26. It will be appreciated that such parts of the system as in FIGURE 1 are located to the right of the terminals 8, 9 and 18 are omitted in the system shown in FIGURE 2. The voltage divider 21, 22 constitutes one branch of a bridge arrangement which is fed with current from a source of alternating current through a transformer 27 which constitutes the other branch of the bridge arrangement, and in this embodiment of the system the indicating member consists of an alternating current amplifier 28 inserted between the tap 20 on the voltage divider 21, 22 and a point 29 located on the secondary winding of the transformer 27 and balanced in relation to the extreme points 8 and 9 of the voltage divider. A source of direct current, such as a battery 12, has its positive pole connected through a high-ohmic resistance 30 to the one outer terminal 8 of the voltage divider, whereas the negative battery pole which may be earthed is in connection with the other outer terminal 9 of the voltage divider. The secondary winding of the transformer 27 is shielded against direct current by means of capacitors 31 and 32. When the system is in operation the source of radiation 4 is adjusted in such manner that the ionization current in the two ionizing chambers 1 and 2 produces leak resistances 25 and 26 of equal magnitude, and in that case the bridge arrangement is in balance since the two capacitors 21 and 22 of the voltage divider are impressed with uniform potentials. If particles of smoke enter the open ionizing chamber 2, the ionizing current in the said chamber will be reduced, whereby the voltage across the one capacitor 22 of the voltage dividerincreases and the voltage across the other capacitor 21 of the voltage divider decreases as a result of which the capacitance of the first-mentioned capacitor is reduced and the capacitance of the last-mentioned capacitor is increased so that the bridge arrangement gets unbalanced and an unbalanced potential occurs across the input terminals of the amplifier 28. As a consequence, a signal relay 19 inserted in the output circuit of the amplifier 28 across a rectifier 33 receives current and gives alarm to indicate that smoke has entered the open ionizing chamber 2.

It will be appreciated that the voltage divider 21, 22 can be built up of several voltage-dependent capacitors if two individual capacitors cannot stand up to the full direct voltage supplied. It will also be appreciated that the direct voltage can be connected to the capacitors 21, 22 through balanced voltage-reducing voltage dividers to ensure that the said capacitors obtain maximum voltage sensitivity.

What I claim and desire to secure by Letters Patent 1. An area monitoring electric alarm system preferably for fire alarms comprising, two metallic ionizing chambers connected together at a common wall, the chambers having registered apertures in the common wall for intercommunication between the interiors of the chambers, one of the chambers having closed walls to the area to be monitored, the other chamber having Walls open to the area to be monitored, the chambers being connected across terminals of a voltage source, an elongated radioactive radiating member disposed within both chambers through the registered apertures the radiating member being displaceable in its longitudinal direction relative to the chambers, a plurality of interconnecting electrodes in both of the chambers, a voltage sensitive indicating member electrically connected to the electrodes, the electrodes being in the form of a metal grid surrounding the radiating member along at least a part of the length of the radiating member, and said common wall mutually separating and electrically insulating the ionizing chambers and the radiating member.

2. An electric alarm system according to claim 1 wherein the elongated radioactive radiating member is a protracted glass tube filled with a radioactive gas radiating beta-particles.

3. An electric alarm system according to claim 1 wherein the metal grid is connected to a tap of a capacitive voltage-dependent voltage divider, one extreme end of which is interconnected to the open ionizing chamber and another extreme end of which is interconnected to the closed ionizing chamber, said voltage divider constituting one branch of a bridge circuit that is fed with electrical energy from a direct voltage source and partly from an alternating voltage source, said alternating voltage source being connected to the extreme ends of the voltage divider through a transformer constituting a second branch of the bridge circuit and having a center terminal between said extreme ends, an amplifier having its input terminals connected to said tap and said center terminal and its output connected to a signal relay for an alarm device.

References Cited by the Examiner UNITED STATES PATENTS 3/1960 Great Britain.

NEIL C. READ, Primary Examiner. D. K. MEYER, Assistant Examiner. 

1. AN AREA MONITORING ELECTRIC ALARM SYSTEM PREFERABLY FOR FIRE ALARMS COMPRISING, TWO METALLIC IONIZING CHAMBERS CONNECTED TOGETHER AT A COMMON WALL, THE CHAMBERS HAVING REGISTERED APERTURES IN THE COMMON WALL FOR INTERCOMMUNICATION BETWEEN THE INTERIORS OF THE CHAMBERS, ONE OF THE CHAMBERS HAVING CLOSED WALLS TO THE AREA TO BE MONITORED, THE OTHER CHAMBER HAVING WALLS OPEN TO THE AREA TO BE MONITORED, THE CHAMBERS BEING CONNECTED ACROSS TERMINALS OF A VOLTAGVE SOURCE, AN ELONGATED RADIOACTIVE RADIATING MEMBER DISPOSED WITHIN BOTH CHAMBERS THROUGH THE REGISTERED APERTURES THE RADIATING MEMBER BEING DISPLACEABLE IN ITS LONGITUDINAL DIRECTION RELATIVE TO THE CHAMBERS, A PLURALITY OF INTERCONNECTING ELECTRODES IN BOTH OF THE CHAMBERS, A VOLTAGE SENSITIVE INDICATING MEMBER ELECTRICALLY CONNECTED TO THE ELECTRODES, THE ELECTRODES BEING IN THE FORM OF A METAL GRID SURROUNDING THE RADIATING MEMBER ALONG AT LEAST A PART OF THE LENGTH OF THE RADIATING MEMBER, AND SAID COMMON WALL MUTUALLY SEPARATING AND ELECTRICALLY INSULATNG THE IONIZING CHAMBERS AND THE RADIATING MEMBER. 